• The U.K.'s initial response to Covid-19, immunization program and world-class research have all played a part in informing other countries how (and sometimes, how not) to deal with the pandemic.
• The U.S. is now following the U.K. which has seen the delta variant become the dominant strain in the country.
• Like the U.K., the U.S. wants to ramp up vaccinations for younger people.

LONDON — Since the start of the coronavirus pandemic, the U.S. has been keeping a close eye on the U.K.

From its initial response to Covid-19 (questioned by many), to its much-praised immunization program and world-class research, all have helped inform how the U.S. — which faced its first major Covid outbreak after Britain — has reacted.

Indeed, the U.K.'s experience of Covid-19 has often been a harbinger of things to come for other countries., including the U.S. It has proved particularly true when it comes to Covid variants which have hit the U.K. and then gone on to dominate globally.

This was first seen with the alpha variant that emerged in southeast England late last year and went on to become dominant across the world. A similar thing then happened with the delta variant that originated in India, but was identified in the U.K. at a relatively early stage.  

Both strains have proved to be far more transmissible than the original coronavirus that first emerged in China in late 2019. The World Health Organization now predicts that the delta variant — which has already spread to more than 80 countries — will become the dominant strain of the disease worldwide.

Read more: The UK's Covid vaccine program and delta surge means it's now a test case for the world

Given the U.K.'s experience of both strains at an earlier stage than many other countries, it is now being seen as something of a "test case" for what could happen in the U.S.

Delta 'greatest threat' to U.S.

The first thing to note is how quickly the delta variant spread across the U.K.

In a relatively short amount of time, the strain supplanted the alpha variant to become dominant in the country (in mid-June delta was responsible for 90% of all infections, a government study showed) — and this happened despite the U.K.'s advanced vaccination rate.

Meanwhile, cases attributed to the delta strain now make up around 20% of newly diagnosed cases in the U.S. according to White House chief medical advisor Dr. Anthony Fauci.

Fauci warned last week that the delta variant is set to become the dominant Covid strain in the country in a matter of weeks, citing the U.K. as precedent. "It just exploded in the U.K. It went from a minor variant to now more than 90% of the isolates in the U.K.," Fauci said on NBC's "TODAY" show.

He said the variant has a doubling time of about two weeks. "So you would expect, just the doubling time, you know, in several weeks to a month or so it's going to be quite dominant, that's the sobering news," he added.

Read more: Fauci says delta accounts for 20% of new cases and will be dominant Covid variant in U.S. in weeks

Fauci had already warned that delta appears to be "following the same pattern" as alpha. "Similar to the situation in the U.K., the delta variant is currently the greatest threat in the U.S. to our attempt to eliminate Covid-19," he said.

In the U.K., infections attributed to delta have spread rapidly among young people and anyone older who has not yet been vaccinated. Similarly, in the U.S., there are concerns that delta could rapidly spread in parts of the South where vaccinations have stalled, NBC News reported Sunday.

The U.S. is not alone in its concern over the delta variant. In Australia, Sydney was put under a two week lockdown over the weekend amid a growing outbreak of the strain. The Australian government's Covid response team is set to meet Monday to discuss the spread of the virus and the possibility of more restrictions.

Vaccination rush

New outbreaks of infections largely blamed on the delta variant have prompted the U.K.'s government to speed up the last leg of its immunization program for people aged 18 and over.

It's hoped that stepping up vaccinations will help stop the wild spread of the strain. Analysis from Public Health England released June 21 showed that two doses of the Pfizer-BioNTech or the Oxford-AstraZeneca Covid-19 vaccines are highly effective against hospitalization from the delta variant.

To date, almost 60% of all U.K. adults have received two doses of the vaccine, while in the U.S., 56% of the population over 18 has been fully vaccinated. The U.K. has not yet authorized Covid shots for adolescents, unlike the U.S. which is giving vaccines to the over-12s.

Read more: Delta Covid variant has a new mutation called 'delta plus': Here's what you need to know

Perhaps wary of how infections have spread in the U.K., the U.S. wants to speed up its vaccinations too. It could take more time than the White House would like, however.

The Biden administration said last Tuesday that it likely won't hit its goal of 70% of American adults receiving one vaccine shot or more by the Fourth of July.

Read more: Covid boosters in the fall? As calls grow for third shots, here's what you need to know

White House Covid czar Jeff Zients said the administration had met its 70% target for people aged 30 and older and is on track to hit it for those aged 27 and older by July Fourth. Zients said U.S. officials were working with state and local leaders to reach younger people.

"We think it'll take a few extra weeks to get to 70% of all adults with at least one shot with the 18- to 26-year-olds factored in," he said.

-CNBC's Nate Rattner and Dawn Kopecki contributed reporting to this story.